Abstract

Large-scale (180 × 60 × 1 mm3) transmission gratings with groove densities of 1250 and 1740 lines/mm have been developed, resulting in diffraction efficiencies above 95%. The throughput of a folded pulse compressor with two large-scale transmission gratings was approximately 80% in a 20-fs Ti:sapphire chirped-pulse amplification (CPA) laser. The parabolic bending of the transmission grating due to anti-reflection (AR) coating was minimized to 2.9 λ at 633 nm by improving the evaporation process. By a simple analysis, we explain why this level of bending does not induce a wavefront distortion through the transmission grating near the Littrow condition while the wavefront from a reflection grating is distorted to nearly twice the bending of the grating. The calculation based on the measured bending shows that both the group delay difference relative to the ideally flat grating from 750 to 850 nm and the spatial pulse front distortion over a 60-mm-diameter input beam are negligible, even when the dispersive beam covers ~140 mm on the grating. The spatial pulse front distortion measured after the compressor was less than the measurement limit (1.5 fs) for a 20-mm-diameter beam, where the beam size in the dispersive direction on the grating was 85 mm.

Figures (12)

(a) Calculated contour map of the efficiencies at 800 nm for the grating with 1740 lines/mm at an incident angle of 44°. (b) and (c) Scanning electronic microscopy (SEM) images of the grating in the scales of 1 μm and 2 μm, respectively. The red bars show the corresponding scales.

(a) Dependence of the diffraction efficiencies on the incident angles for the TE polarization for a 1740-lines/mm grating. Maximum efficiency over 94% appears at the angle of 44°. The inset shows the normalized spectrum of the probe pulse for the measurement. (b) Dependences of the diffraction efficiency on the wavelength for the TE polarization in 1740- (red) and 1250- (black) lines/mm gratings at incidence angles of 44° and 30° respectively. The inset shows a typical spectrum of the probe centered at 800 nm.

(a) Reflected wavefront map of a grating with a groove density of 1250 lines/mm at 633 nm after optimizing the AR coating (double pass). (b) Wavefront profile (red solid line) averaged from y = 27.5–32.5 mm and fitted to a 3rd-order polynomial curve (green dash line). The blue line is the deviation from the fitted curve.

Wavefront measurement of (a) the diffracted beam double-passed through a transmission grating and (b) diffracted beam reflected from a transmission grating. (c) Wavefront error in λ at 633 nm versus grating position along the dispersive direction. At the Littrow angle (23.3°), the wavefront is flat through the transmission grating (the green short-dashed line), while highly parabolic in the reflection grating (the blue dash-dotted line). The incidence angles are shown in parentheses. At the off Littrow angles, the wavefront through the transmission grating depends on incident angle and changes the sign. PV = 52 λ in H(x).

Notations for the wavefront analysis. (a) Diffraction by a transmission grating; (b) diffraction by a reflection grating. H(x) is distorted grating surface measured by the deviation from the ideally flat surface, where x is the distance along the dispersive direction. W(x') is the distorted wavefront after diffraction. α0 and β0 are the incidence and diffraction angles, respectively. Δα and Δβ are the changes of incidence and diffraction angles due to the distortion of the grating, respectively.

Notation for the ray-tracing analysis. Pin: input plane, Gi: i-th grating, Pout: reference plane. Note that the curvatures of gratings are illustrated as positive for G1 and G2, and negative for G3 and G4, respectively.

Group delay differences with respect to the flat grating pairs for three different beam positions (red: x = 30 mm; black; x = 0 mm; blue: x = −30 mm). The solid curves show the relative group delays directly obtained by ray-tracing, while the dotted curves show those after subtracting the third-order dispersion. The upper panels (a, b) and lower ones (c, d) are for the cases with 1250- and 1740- lines/mm gratings, respectively. The insets illustrate the directions of the grating curvature.

Schematic of the folded compressor with two gratings in a Ti:sapphire CPA laser with two transmission gratings (1250 lines/mm). Sizes are 40 × 60 × 1 mm3 and 180 × 60 × 1 mm3, respectively. The input beam size is about 20 mm, and the dispersive length on the transmission grating 2 is about 85 mm.

Results of the spatial pulse front distortion in the horizontal (a) and vertical direction (b). The RMS errors were 0.44 μm (horizontal) and 0.53 μm (vertical). The path difference mainly originates from the noise of the laser system.